There is a continuing need in the printed circuit, graphic arts, and related industries to transfer images photographically from original (positive or negative) photomasks to one or both sides of a light sensitive sheet. In many important cases, these images must be aligned or registered precisely to locations previously established on the element. In the case where images are transferred to both sides of the element, it is often further required that the front and rear images also be registered precisely to each other. In all cases, the photographic operation is carried out in a manner which maintains, to the greatest precision possible, both the definition and the relative locations of the features of the images, while minimizing the transfer of undesired or spurious features.
Printed circuits are often prepared by a repetitive image transfer process using dry film photoresists. The equipment used to practice the photoresist process has consisted in general of discrete pieces such as a cleaner or scrubber for cleaning the copper-clad substrate boards, an oven for preheating the boards, a roll laminator applying heat to the film and board as it laminates them together, an actinic radiation exposure station, and solvent wash-out apparatus for developing a resist image on the copper substrate. Such processes and equipment are fully described in Coombs, "Printed Circuits Handbook", McGraw-Hill Second Edition (1979) and in DeForest, "Photoresist Materials and Processes", McGraw-Hill (1975).
Typically, manual transfer and positioning of the substrate board occurs between each piece of equipment which increases expense and lessens reproducibility, leading to yield loss in the process.
Various attempts have been made to automate the photoresist process but automation has been adopted to only a limited extent. Recently, however, board trimming has been successfully incorporated into an integrated high productivity cleaning and laminating system as disclosed in U.S. Pat. No. 4,293,635, European Patent Application No. 81104013.8, and U.S. Ser. No. 153,636 and U.S. Ser. No. 153,637, both filed May 27, 1980. More recently a high productivity registration and exposure process has been disclosed in U.S. Ser. No. 387,534 filed June 11, 1982. These advances in automation results in substantial cost savings to the user.
A process for coating a photosensitive liquid onto a substrate simultaneously with photomask registration and exposure has been disclosed in U.S. Ser. No. 387,535 filed June 11, 1982. This process is particularly useful for applying permanent coatings to circuit boards before soldering operations. Although excellent protection is obtained for thicker coated layers, similar protection by thinner coated layers is more difficult to accomplish. Since thinner coatings are desirable, a need exists for insuring an adequate coated thickness over the printed circuit pattern.